Hydraulic systems are nowadays used in a plethora of technical applications.
In the beginning of hydraulic applications, mostly hydraulic cylinders were used to move heavy weights with high forces. Well known examples are doors for locks, lifting devices for the shovel of a wheel loader, for the fork of a fork-lift truck or for the trough of a dump truck.
However, hydraulic systems have evolved from these basic systems and more and more hydraulic applications have become common. For example, hydraulic systems are nowadays even used as power transmitting devices. The power output of a combustion engine drives a hydraulic pump. The hydraulic fluid, pumped by the hydraulic pump, is led to a hydraulic motor through hydraulic tubes. There, the pressure energy of the hydraulic fluid is converted back to mechanical movement. With increasing efficiencies, hydraulic systems become more and more competitive to traditional power transmissions. However, there are still problems involved with current hydraulic systems. For instance, one major disadvantage is the price for hydraulic systems.
The price problem becomes even stronger, if highly efficient pumps, such as synthetically commutated hydraulic pumps are used. Synthetically commutated hydraulic pumps are also known as digital displacement pumps. They are a unique subset of variable displacement pumps. A basic design is described in U.S. Pat. No. 5,190,446, EP-A-0361927 or US 2006-039795 A1, for example. Such synthetically commutated hydraulic pumps are in many ways superior to traditional hydraulic pumps. For instance they have a higher efficiency and they are more flexible when in use. For example, their fluid flow output can be changed easily by an appropriate actuation of the inlet (and in some cases even the outlet) valve of the synthetically commutated hydraulic pump. With an appropriate design and an appropriate actuation of the electrically actuatable valves, a reverse pumping mode and/or a motoring mode can be achieved as well for the synthetically commutated hydraulic pump.
However, synthetically commutated hydraulic pumps have short-comings as well. One of the chief shortcomings in the field of synthetically commutated hydraulic pumps is the usually high cost of synthetically commutated hydraulic pumps, when compared to the cost of traditional hydraulic pumps. Another problem is the fact, that synthetically commutated hydraulic pumps are normally physically larger for a given power unit displacement than conventional hydraulic pumps. Still another problem with synthetically commutated hydraulic pumps is that normally a significant amount of electrical power is required to rapidly and frequently actuate the actuated valves.
Moreover, synthetically commutated hydraulic pumps show their intrinsic technical advantages, when it comes to providing high pressures at relatively low flow rates. On the contrary, when there is a need for a cost effective pump that produces high hydraulic fluid flow rates at relatively low system pressures, synthetically commutated hydraulic pumps have been impractical so far. Therefore, in quite a lot of applications, traditional hydraulic pumps are still used, in spite of the availability of synthetically commutated hydraulic pumps. Admittedly, this is an acceptable work around in applications, where there is solely a demand for high hydraulic fluid flow at relatively low pressures. In applications, however, where there is at least during certain time intervals a demand for high pressures as well as for high flowrates at relatively low pressures, there is still no convincing solution so far. This is a big issue, because a large portion of todays hydraulic applications have exactly this type of hydraulic fluid demand. If you think of a wheel loader or a fork-lift truck, you have a need for a high hydraulic fluid flow rate at a low pressure, when the vehicle is to be moved by a hydraulic motor at higher speeds on plane grounds (e.g. when driving on a road). On the other hand, if you want to lift a heavy load with the lifting hydraulics of a fork-lift truck or a wheel loader, you have a need for hydraulic fluid at high pressures, whereas a low fluid flow rate is acceptable. The same situation can arise, if you have to drive the vehicle with a heavy load up a steep incline.
One traditional way to cope with this problem would be to provide a high pressure pump of a large size, so that the high pressure pump can provide a large fluid flow output. However, this approach is not very cost effective.
Another text book approach for such a situation is to provide for a parallel arranged high pressure pump and a high volume low pressure pump. Whereas the high pressure pump is always connected to the hydraulic consumer, the high volume low pressure pump is connected to the hydraulic consumer side via a check valve, which opens only, if the pressure on the hydraulic consumer side is sufficiently low. A big problem with such parallely arranged pumps is the controllability of the fluid output flow. According to the state of the art, both high pressure and low pressure pumps are pumping under all conditions at maximum pumping rate. If the fluid flow demand of the consumers is lower than the fluid output flow of the pump arrangement, any excess fluid flow is simply dumped back into the hydraulic fluid reservoir via pressure relief valves. While such arrangements work well, their energy efficiency is usually unsatisfactorily low. Especially under low fluid flow conditions, energy is wasted by first raising the pressure of hydraulic fluid and then dumping said fluid right afterwards without performing any useful work. The design however, is necessary to provide for a smooth transition, particularly in the transition area, when the fluid output flow of the high volume low pressure pump starts in or fades out, respectively. An additional problem with such a system is that it is normally incapable of providing low pressure flow at low flow rates without additional system complexity because the check valve is in the low pressure pump flow below a certain pressure level, not based on a flow demand.